Aircraft usually have several movable control surfaces attached to the trailing and/or leading edges of a wing that are used to fulfil different functions, for example increasing wing maximum lift. Amongst the numerous effective means that increase wing maximum lift, movable wing leading and trailing edge control surfaces may include for example single slotted leading edge slats (generally denoted as “slats” hereinforth) and single or multiple slotted trailing edge flaps (generally denoted as “flaps” hereinforth) which are employed in many aircraft models.
These high lift devices are usually controlled manually by the cockpit crew by means of a slats/flaps (S/F) lever. The lever can usually be set into discrete positions related to predefined S/F configurations. In case of large transport aircraft, slats and flaps are usually moved by means of hydraulically or electrically powered actuators. For these type of systems electrical control signals are provided from the S/F lever to a control device which in turn sends control signals to the actuators that move the slats and/or flaps into the commanded positions.
Normally, slats and/or flaps are extended before the take-off run, for holding flight and for the approach and landing flight phases. High lift devices are usually retracted after the initial climb phase following the take-off or go-around in order to reduce drag as well as after landing. Hence, slats and/or flaps are held in a retracted position in climbing and cruise flight phases as well as during ground operation (taxiing, parking). The increased maximum lift coefficient during take-off and initial climb phases on the one hand and during approach and landing flight phases on the other hand allows reduced flight speed and enhances aircraft performance, for example by allowing increased payload depending on runway length, ambient atmospheric conditions and similar external influence parameters.
The optimum aircraft speed for retraction of high lift devices usually differs from the optimum aircraft speed for extension of the respective high lift devices. This is mainly due to there being limited allowed speed ranges for each configuration of a high lift device. As such, the speed ranges of adjacent configurations of a high lift device need to at least partially overlap in order to allow for efficient configuration control of the high lift device. The extent of the operational allowed speed ranges, the aerodynamic and structural performance of the high lift devices as well as the required degree of performance optimization may be considered to determine the required number of configurations for high lift devices. As an example, the single-aisle jetliner passenger aircraft A320 of Airbus has 6 discrete configurations for high lift devices, termed “0”, “1”, “1+F”, “2”, “3” and “Full”.
Occasionally leading edge high lift devices may not only be used to increase the margin between the actual and the maximum lift coefficient, that is in a planned deployment mode, but they can also be used to prevent the wing from stalling, if the actual lift coefficient gets close to the maximum lift of the wing, that is in an ad hoc deployment mode. For example, leading edge devices may be automatically deployed in the latter case once a maximum acceptable angle of attack has been reached.
Such an automatic system for ad hoc deployment is for example described in document DE 639,329 A. The leading edge slats according to this document are directly actuated by fluid forces so the slats are deployed once an angle of attack greater than the extension angle has been reached, while the devices are going to be retracted when the angle of attack is reduced to values corresponding to a retraction angle of attack. Airspeed dependent automatic functions may retract slats and particularly flaps at high speed in order to prevent the devices from overload such as for example disclosed in the documents U.S. Pat. No. 2,350,751 A, DE 25 31 799 C3, U.S. Pat. No. 4,042,197 A and EP 1 684 144 A1. In these documents, configurations of high lift devices are controlled for example by airspeed, dynamic pressure and/or distance from a defined point on the ground. Further speed dependent high lift device setting functions featuring separate take-off and approach modes for enhanced aircraft performance during high lift device retraction and deployment phases are disclosed in the document DE 10 2007 045 547 A1.